1. Field of the Invention
The invention discloses a method for the iterative determination of the distance between a receiving station and a transmitting station and also a corresponding calculating unit, a corresponding computer software product and a corresponding data carrier.
2. Description of the Related Art
The GPS method (GPS: Global Positioning System) for determining the position of a station is based on the principle of measuring signal transfer times between satellites having known positions and a GPS receiver that is installed in a station. The transfer time for a signal is proportional to the distance of the respective satellite from the station, that is, the distance between a satellite and the station can be converted, using the propagation velocity of the corresponding signal, into the respective signal transfer time. Since both the position of the station and the extent by which the clock at the station deviates from the atomic clocks on the synchronized satellites are unknown, the position of the station is determined using the distances from at least four satellites.
The signals which the satellites transmit are satellite-specific code sequences which are modulated to a carrier frequency. The basic measurement of the station to determine its position consists in measuring for each satellite a phase offset between the respective code received and a copy of the code, which is produced in the receiver. The respective phase offset (chip code offset) is a measure of the signal transfer time and therefore of the distance between the station and the corresponding satellite. A code available for civil use currently consists of 1023 chips and is repeated with a cycle of one millisecond. In the one-millisecond duration of a code cycle, a satellite signal which is propagated in a vacuum at the speed of light travels about 300 km. The distance between the satellite and the ground station is typically several thousand kilometers, however. During the time which the signal requires to cover the distance between the satellite and the station, however, the code is therefore repeated several times. In other words, this means that, during the signal transfer time, the satellite uses the code N times to modulate the continuously transmitted signal and that the station receives the code N times in the same period of time.
A precise determination of the phase offset is an essential component of a determination of the distance that the station is from the respective satellite and therefore forms the basis of the determination of the station's position. Since the phase offset is, however, measured using only one code cycle, the total transfer time that the signal requires to cover the distance between the satellite and the station cannot be determined from the phase offset alone. The signal transfer time is made up of a whole number of repeats of the code and of a modulo ensuing from the determination of the phase offset. The phase offset is calculated directly by the station, whereas the integral multiples of the code are calculated by an iterative method in the course of the determination of the station's position. The computing power that such an iterative method needs is directly dependent on the number of iterative steps required to determine the integral multiples in a satellite's code.